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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
324 am EDT Sat Jul 20 2019

Synopsis...
high pressure will build over the southeastern United States
through the weekend. A cold front will approach from the Great
Lakes Sunday and Monday before passing through our area Monday
night into Tuesday. High pressure will return for the middle
portion of next week.

&&

Near term /through tonight/...
surface high pressure will remain draped across the Atlantic
Ocean into the southeastern Continental U.S. While upper-level high
pressure continues to settle overhead. This combination will
continue to bring very hot and humid conditions to the area.
Low-level temps are progged to be around 1-2c higher this
afternoon compared to Friday, and dewpoints fairly similar to
Friday. Max temps will range from the 90s across most locations
west of the Blue Ridge (cooler along the ridge tops) to the
upper 90s/lower 100s east of the Blue Ridge mountains. This will
cause heat indices to top off around 100 to 115 degrees during
peak heating late this morning into early this evening across
the area. The highest heat indices will be near and east of
Interstate 95. An excessive heat warning remains in effect
for all locations east of the Blue Ridge mountains, the eastern
Panhandle of West Virginia, portions of north-central Maryland,
and portions of the northern Shenandoah Valley in Virginia. A
heat advisory is in effect for the central Shenandoah Valley
into the valleys of the Potomac Highlands. A popup thunderstorm
cannot be completely ruled out across northeastern Maryland
where weak shortwave energy may pass through from a weakening
mesoscale convective vortex to our north and across the Potomac Highlands of West
Virginia and Virginia from terrain circulation. However, any
coverage will be isolated at best and much of the time will be
dry. Therefore, this will likely not provide any relief from the
heat.

Upper-level high pressure will remain overhead tonight, causing
very warm and humid conditions. Min temps will range from the
70s for most areas to the lower and middle 80s in downtown
Washington and Baltimore. Heat indices for urban areas along the
I-95 corridor may hold in the 90s.

&&

Short term /Sunday through Monday night/...
the upper-level high will remain overhead Sunday while surface
high pressure remains draped across the southeast Continental U.S. Into
the Atlantic. More very hot and humid conditions are expected.
Max temps will be well into the 90s for most areas to perhaps
near 100 degrees with dewpoints in the 70s for many locations.
This will likely cause heat indices to top off around 100 to 115
degrees. However, the ridge may break down just a bit later
Sunday into Sunday night as a cold front drops through the
Midwest and the upper-level trough begins to dig through the
northern plains. As the ridge breaks down a little, this will
cause a southwest flow to develop at the mid-levels, and this
southwest flow will usher in some more moisture into the area.
This combined with a weakening capping inversion from height
falls may be enough to trigger some showers and thunderstorms
during the afternoon and evening hours. Latest thinking is that
coverage will be isolated to scattered since there will not be a
strong lifting mechanism, but with this type of heat and
humidity any storms that do develop may become severe.

Will continue with the excessive heat watch for the area late
Sunday morning into Sunday evening. Given the increased chance
for convection, latest thinking is that we are not categorically
confident for the highest heat indices to be realized, but it
does appear more likely than not that it will. Will re-assess
later today and tonight to see if an upgrade to a warning is
necessary.

The cold front will continue to drop south approaching the Ohio
Valley Sunday night, and height falls will continue as a result.
Therefore, showers and thunderstorms are possible during the
overnight hours as well. Some may be severe.

The cold front will pass through the Ohio Valley Monday and into
our area Monday night. Lower heights and a southwest flow aloft
combined with forcing from the approaching front and a pressure
trough overhead will be enough to trigger showers and
thunderstorms, especially during the afternoon and evening
hours. Increased shear profiles along with the high instability
suggests that storms will have the chance to be severe. Monday
will not be as hot as recent days due to more cloud cover and
more coverage of showers and thunderstorms, but Max temps in the
80s and 90s with high humidity will still make it feel quite
hot with heat indices around 100 degrees for locations near and
east of Interstate 95. More showers and thunderstorms are
possible Monday night as the cold front moves into the area.

&&

Long term /Tuesday through Friday/...
surface front may linger near enough to the area on Tuesday to
result in continued showers and a few thunderstorms, though
instability should be generally less than Monday. Depending on
the exact positioning of the front, some heavy rainfall may
still be possible especially over southern Maryland into central
Virginia, but not as high probability or widespread as Monday.
With cloud cover and precipitation around on Tuesday, high
temperatures will be notably cooler than previous days, and may
struggle to get out of the 70s in some areas.

Thereafter, high pressure and much milder temperatures and
humidity move into the region. Temperatures should average in
the 80s each afternoon with lows in the 60s overnight.

&&

Aviation /07z Saturday through Wednesday/...
VFR conditions are expected most of the time through Sunday
night. A thunderstorm is possible later Sunday afternoon through
Sunday night, and some may produce gusty winds and brief subvfr
conditions. Showers and thunderstorms are expected Monday and
Monday night, especially during the afternoon and evening hours.
Some storms may be severe.

Sub-VFR possible Tuesday with lingering chances for
showers and a few thunderstorms, but generally less overall from
Monday.

&&

Marine...
high pressure to the south will likely cause a west to southwest
flow through Sunday and wind speeds should remain below Small Craft Advisory
criteria for most areas. Isolated to scattered storms are
possible later Sunday afternoon and Sunday night. Some storms
may produce gusty winds.

A cold front will approach the waters Monday before passing
through Monday night into Tuesday. A Small Craft Advisory may be
needed for portions of the waters during this time. Showers and
thunderstorms are also expected during this time, and some
storms may be severe.

Sub-Small Craft Advisory conditions likely Wednesday.

&&

Climate...
here are the warm temperature records for this weekend:

Record daily high temperatures
Jul 20 Jul 21
dca 106 (1930) 104 (1926)
BWI 102 (1930) 104 (1930)
iad 101 (1980) 101 (1991)

Record daily warm low temperatures
Jul 20 Jul 21
dca 82 (2015) 82 (1987)
BWI 80 (1930) 83 (1930)
iad 75 (2015) 77 (1987)

Highest July temperatures
highest Max warmest low
dca 106 (7/20/1930) 84 (7/24/2011, 7/23/2011, 7/16/1983)
BWI 107 (7/10/1936) 83 (7/21/1930)
iad 105 (7/22/2011) 78 (7/24/2010, 7/8/2010)

All-time highest temperatures
highest Max warmest low
dca 106 (7/20/1930, 8/6/1918) 84 (7/24/2011, 7/23/2011, 7/16/1983)
BWI 107 (7/10/1936) 83 (8/5/1930, 7/21/1930, 6/6/1925)
iad 105 (7/22/2011) 79 (8/8/2007)

Longest stretches of consecutive days with high temperatures at or
over 100 degrees at iad:
2 (7/7-8/2012, 7/21-22/2011, 7/6-7/2010, 8/16-17/1997, 7/16-17/1988
and 7/20-21/1980)

Longest stretch of consecutive days with low temperatures at or over
80 degrees at dca:
5 (7/16-20/2013)

Longest stretch of consecutive days with low temperatures at or over
80 degrees at bwi:
4 (8/8-11/1900)

Temperature records for the Washington District of Columbia area have been kept at
what is now Ronald Reagan Washington National Airport (dca) since
1941. Additional temperature records observed downtown extend the
period of record back to 1872.

Temperature records for the Baltimore Maryland area have been kept at what
is now Baltimore-Washington international thurgood Marshall Airport
(bwi) since 1950. Additional temperature records observed downtown
extend the period of record back to 1872.

Temperature records for the Sterling/Dulles Virginia area have been kept
at what is now Washington Dulles International Airport (iad) since
1960.

&&

Lwx watches/warnings/advisories...
District of Columbia...excessive heat warning from 11 am this morning to 9 PM EDT
this evening for dcz001.
Excessive heat watch from Sunday morning through Sunday
evening for dcz001.
Maryland...excessive heat warning from 11 am this morning to 9 PM EDT
this evening for mdz003>006-011-013-014-016>018-503>508.
Excessive heat watch from Sunday morning through Sunday
evening for mdz003>006-011-013-014-016>018-503>508.
Heat advisory from 11 am this morning to 9 PM EDT this evening
for mdz502.
Virginia...excessive heat warning from 11 am this morning to 9 PM EDT
this evening for vaz028>031-036>040-050>057-501-502-505-
506.
Excessive heat watch from Sunday morning through Sunday
evening for vaz028>031-036>040-050>057-501-502-505-506.
Heat advisory from 11 am this morning to 9 PM EDT this evening
for vaz025>027.
WV...excessive heat warning from 11 am this morning to 9 PM EDT
this evening for wvz051>053.
Excessive heat watch from Sunday morning through Sunday
evening for wvz051>053.
Heat advisory from 11 am this morning to 9 PM EDT this evening
for wvz050-055-502-504.
Marine...none.

&&

$$
Synopsis...bjl
near term...bjl
short term...bjl

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